Field of the Invention
The present invention concerns techniques for magnetic resonance thermometry and a magnetic resonance system for implementing such techniques. In particular, the invention concerns techniques for thermometry based on phase shifts in acquired MR data that take into account different reference phases for different regions of an examined person.
Description of the Prior Art
In magnetic resonance (MR) imaging, structures and/or parameters of an examined person are imaged. For this purpose, a magnetization of protons is deflected out of the steady state (typically parallel to a basic magnetic field) by radiation of a radio-frequency (RF) pulse. The excited transverse magnetization oscillates and can be measured by means of inductive measurement techniques. The MR data acquired in such a manner depict the structures and/or properties.
It is possible to implement MR imaging such that the contrast in the MR data or in MR images is indicative of a temperature (MR thermometry). For example, MR thermometry is based on the physical effect that the proton resonance frequency (PRF) shows a dependency on the temperature. Typically this is a linear dependency. Therefore, a transverse magnetization excited within the scope of MR thermometry shows a corresponding dependency of the acquired phase on the temperature. Phase shifts are typically measured between a measurement phase and a reference phase; for example, see in this regard Equation 16 from “MR Thermometry” by V. Rieke and K. B. Pauly in J. Mag. Reson. Med. Imag. 27 (2008) 376-390.
Techniques are known that obtain the reference phase from reference MR data that represent an image an examined person, for example at a known reference temperature before the introduction of heat (reference-based MR thermometry). In other words, a “historic” reference MR image is used to determine the temperature. For example, see the aforementioned publication by V. Rieke.
Techniques are also known that obtain the reference phase from the same MR data from which the measurement phase is also obtained, but from a different imaged region, for example (reference-free MR thermometry). See for example R. Salomir et al., Proc. Intl. Soc. Mag. Reson. Med. 18 (2010) 247.
Different applications or imaged regions respectively have advantages and disadvantages relative to these aforementioned techniques of MR thermometry. It is frequently not possible, or only possible to a limited extent, to image different items equally well with both techniques.
In particular, periodic movements, for example translation, rotation, expansion, compression, etc., of organs and body regions on a characteristic time scale of seconds to minutes (inter-fraction motion) can hinder the application of reference-based MR thermometry. This is the case since a significant movement can already have occurred between the acquisition of the MR data which are used to determine the reference phase and the acquisition of the MR data which are used to determine the measurement phase, and there is thereby no or only a slight phase coherence between the two MR data.
The use of reference-free thermometry can be possible only to a limited extent if the phase coherence is spatially limited, meaning that a variation of the phase over the location occurs due to susceptibility fluctuations, for example.
A need therefore exists for improved techniques of MR thermometry which enable a particularly precise measurement of the temperature.